214 A. V. LEBEDINSKY, V. M. MASTRYUKOVA AND A. D. STRZHIZHOVSKY 



Tlie theoretical curve giving the mitotic index of the irradiated 

 population as a function of the time and radiation dose is presented in 

 Fig. 1. 



Fig. 1. — Radiation damage to mitotic activity. 



The mitotic activity after radiation reduces in an exjjonential 

 fashion, its period amounts to t„i (mitosis duration). The initial value 

 of the mitotic index is determined b}^ the number of cells that have 

 avoided damage to the genetic apparatus immediately during the 

 mitosis stage. The minimum value of the mitotic activity is estimated 

 by the number of cells which avoided both biochemical damage and 

 damage to genetic structures. 



The intensity of restoration is determined by the value a + K. 



The form of the restoration curve depends upon the irradiation dose 

 or upon the rate of cell damage (Fig. 1). 



At a small dose of irradiation when the intensity of cell restoration a 

 is much greater than the intensity of cell renovation, the form of the 

 restoration curve is near to the exponential. 



The curve will have an S shape if the irradiation dose is great and 

 cellular renovation forms the very basis of the restoration of cellular 

 activity. 



An analogous restoration curve was obtained by Powers (1955), but 

 the ground of his considerations was somewhat different from ours. 

 The author regarded as the object of restoration not the cell, as we did, 

 but a certain intracellular substrate the content of which in the cell 

 determines the intensity of cell division. Though, according to the 

 author, the restoration rate of this substrate does not depend on irradia- 

 tion dose, the probability of the restoration of the cell's mitotic 

 activity depends (as it does in our case) on the radiation dose in view 

 of the relationship between the quantity of damaged material inside the 

 cell and the radiation dose. 



